Wireless transmitting and receiving device and method

ABSTRACT

A wireless device, method, and signal for use in communication of a wireless packet between transmitting device and a wireless receiving device via a plurality of antennas, wherein a signal generator generates wireless packet including a short-preamble sequence used for a first automatic gain control (AGC), a first long-preamble sequence, a signal field used for conveying a length of the wireless packet, an AGC preamble sequence used for a second AGC to be performed after the first AGC, a second long-preamble sequence, and a data field conveying data. The AGC preamble sequence is transmitted in parallel by the plurality of antennas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/018,251 and is based upon and claims the benefit of priority fromprior Japanese Patent Applications No. 2003-433347, filed Dec. 26, 2003;and No. 2004-357097, filed Dec. 9, 2004, the entire contents of each ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless transmitting device andwireless receiving device for respectively transmitting and receivingradio signals in mobile communication system like a wireless LAN, usinga wireless packet including a preamble and data, and a wirelesstransmission method and wireless receiving method for use in thedevices.

2. Description of the Related Art

The Institute of Electrical and Electronics Engineers (IEEE) is nowdefining a wireless LAN standard called IEEE 802.11n, which aims toachieve a high throughput of 100 Mbps or more. It is very possible thatIEEE 802.11n will employ a technique, called multi-input multi-output(MIMO), for using a plurality of antennas in a transmitter and receiver.IEEE 802.11n is required to coexist with the standard IEEE 802.11a whereOFDM (Orthogonal Frequency Division Multiplex) is used. So, it isrequired that IEEE 802.11n wireless transmitting device and receivingdevice have so called backwards compatibility.

A proposal presented by Jan Boer et al. in “Backwards Compatibility”,IEEE 802.11-03/714r0, introduces a wireless preamble for MIMO. In thisproposal, a short-preamble sequence used for time synchronization,frequency synchronization and automatic gain control (AGC), along-preamble sequence used to estimate a channel impulse response, asignal field indicating a modulation scheme used in the wireless packet,and another signal field for IEEE 802.11n are firstly transmitted from asingle particular transmit antenna. Subsequently, long-preamblesequences are transmitted from the other three transmit antennas. Afterfinishing the transmission of the preamble, transmission data istransmitted from all the antennas.

From the short-preamble to the first signal field, the proposed preambleis identical to the preamble stipulated in IEEE 802.11a where singletransmit antenna is assumed. Therefore, when wireless receiving devicesthat conform to IEEE 802.11a receive a wireless packet containing theBoer's proposed preamble, they recognize that the packet is based onIEEE 802.11a. Thus, the proposed preamble conforming to both IEEE802.11a and IEEE 802.11n enables IEEE 802.11a and IEEE 802.11n tocoexist.

Generally, in wireless receiving devices, demodulation of a receivedsignal is performed by digital signal processing. Therefore, ananalog-to-digital (A/D) converter is provided in the devices fordigitizing a received analog signal. A/D converters have an inputdynamic range (an allowable level range of analog signals to beconverted). Accordingly, it is necessary to perform automatic gaincontrol (AGC) for adjusting the levels of received signals within theinput dynamic range of the A/D converter.

Since the estimation of a channel impulse response using theabove-mentioned long preamble sequences is performed by digital signalprocessing, AGC must be performed using the signal transmitted beforethe long-preamble sequence. In the Boer's preamble, AGC is performedusing a short-preamble sequence transmitted before the long-preamblesequence from a particular transmit antenna. That is, the receivinglevel of the short-preamble sequence is measured, and AGC is performedso that the receiving level falls within the input dynamic range of theA/D converter. By virtue of AGC using the short-preamble sequence, thelong-preamble sequence and data transmitted from the particular transmitantenna can be received correctly. If all the antennas are arrangedapart, the receiving levels of signals transmitted from the antennas areinevitably different from each other. Therefore, when a wirelessreceiving device receives long-preamble sequences transmitted from theother three transmit antennas, or data transmitted from all theantennas, their receiving levels may be much higher or lower than thelevel acquired by AGC using the short-preamble sequence transmitted fromthe particular transmit antenna. When the receiving level exceeds theupper limit of the input dynamic range of the A/D converter, the outputof the A/D converter is saturated. On the other hand, when the receivinglevel is lower than the lower limit of the input dynamic range of theA/D converter, the output of the A/D converter suffers a severequantization error. In either case, the A/D converter cannot performappropriate conversion, which adversely influences the processing afterA/D conversion.

Further, data is transmitted from all the antennas. Therefore, duringdata transmission, the range of variations in receiving level is furtherincreased, which worsens the above-mentioned saturation of the A/Dconverter output and/or the quantization error therein, therebysignificantly degrading the receiving performance.

As described above, in the Boer's proposed preamble, AGC is performed atthe receive side using only the short-preamble sequence transmitted froma single transmit antenna, which makes it difficult to deal withvariations in receiving level that may occur when signals transmittedfrom the other antennas in MIMO mode are received.

BRIEF SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, there is provided awireless transmitting device for use in communication with a wirelessreceiving device with a wireless packet, comprising: a plurality ofantennas; and a signal generator generates a signal for the wirelesspacket being transmitted, the wireless packet comprising: ashort-preamble sequence used for a first automatic gain control (AGC); afirst long-preamble sequence; a signal field used for conveyinginformation regarding a length of the wireless packet; an AGC preamblesequence used for a second AGC to be performed after the first AGC; asecond long-preamble sequence; and a data field conveying data, whereinthe AGC preamble sequence being transmitted by the plurality of antennasin parallel.

Since a signal format employed in the invention includes preambles forfine tune the AGC for MIMO reception transmitted from multiple antennas,the input level of an A/D converter can be appropriately adjusted with ashort time, thereby enhancing the receiving performance of a wirelessreceiving device and reducing the number of resolution bits of the A/Dconverter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the invention,and together with the general description given above and the detaileddescription of the embodiment given below, serve to explain theprinciples of the invention.

FIG. 1 is a view illustrating a format for a wireless packet includingthe AGC preambles for wireless communication used in an embodiment ofthe invention;

FIG. 2 is a block diagram illustrating the configuration of a wirelesstransmitting device according to the embodiment;

FIG. 3 is a block diagram illustrating the configuration of a wirelessreceiving device according to the embodiment;

FIG. 4 is a block diagram illustrating a configuration example of areceiving unit incorporated in the device of FIG. 3;

FIG. 5 is a graph illustrating the distribution of the receiving powerof short preambles and data in the prior art;

FIG. 6 is a graph illustrating the distribution of the receiving powerof short preambles and data in the embodiment;

FIG. 7 is a block diagram illustrating another configuration example ofthe receiving unit;

FIG. 8A is a flowchart in explaining the operation of a gain controller;

FIG. 8B is a flowchart showing a first AGC operation and second AGCoperation.

FIG. 9 is a block diagram illustrating a wireless receiving deviceaccording to a modification of the embodiment;

FIG. 10 is a block diagram illustrating a configuration example of areceiving unit incorporated in the wireless receiving device of FIG. 9;

FIG. 11 is a block diagram illustrating a configuration example of thepropagation path estimation unit appearing in FIG. 3;

FIG. 12 is a view illustrating structural examples of the AGC preamblesappearing in FIG. 1;

FIG. 13 is a view illustrating other structural examples of the AGCpreambles appearing in FIG. 1; and

FIG. 14 is a view illustrating a wireless transmitting device accordingto another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will be described in detail with referenceto the accompanying drawings.

FIG. 1 shows a format for a wireless packet employed in a firstembodiment of the invention. This format is a physical layer protocoldata unit format for the MIMO mode and provides interoperability andcoexistence with IEEE802.11a wireless stations.

As seen from FIG. 1, a preamble includes a physical layer convergenceprotocol (PLCP) signal transmitted from an antenna Tx1. The PLCP signalincludes a short-preamble sequence 101, first long-preamble sequence102, first signal field (SIGNAL) 103 and second signal field (SIGNAL 2)104. The short-preamble sequence 101 contains several unit preambles SP.The long-preamble sequence 102 contains the unit preambles LP havingrespective predetermined lengths. The preambles LP are longer than thepreambles SP.

The short-preamble sequence 101, first long-preamble sequence 102 andfirst signal field 103 conform to IEEE 802.11a, while the second signalfield 104 is necessary for the new wireless LAN standard IEEE 802.11n.First signal field 103 conforming to IEEE 802.11a may be called “legacysignal field”. Since the second signal field 104 is provided for newhigh throughput wireless LAN standard, it may be called “high throughputsignal field”. A guard interval GI is inserted between theshort-preamble sequence 101 and the long-preamble sequence 102.

After the PLCP signal, AGC preambles 105A to 105D that are transmittedin parallel from a plurality of antennas Tx1 to Tx4 are positioned. TheAGC preambles 105A to 105D are transmitted simultaneously from aplurality of antennas Tx1 to Tx4. The AGC preambles 105A to 105D areused to enable the receiving device to perform fine AGC when performingMIMO communication. These preambles are unique to perform fine tune theAGC for reception of MIMO mode in accordance with IEEE802.11n.Therefore, the AGC preambles 105A to 105D may be called “high throughputshort trainings field”. On the other hand, since the short-preamblesequence 101 conforms to IEEE 802.11a, being used for coarse AGCoperation, it may be called “legacy short training field”.

After the AGC preambles 105A to 105D, second long-preamble sequences106A to 109A, 106B to 109B, 106C to 109C and 106D to 109D arepositioned. In the embodiment, the same signal sequences are used as theAGC preambles 105A to 105D. However, different signal sequences may beused as the AGC preambles 105A to 105D. A guard interval GI is insertedbetween each pair of adjacent ones of the unit preambles LP that formthe second long-preamble sequences 106A to 109A, 106B to 109B, 106C to109C and 106D to 109D. As described later, the second long-preamblesequences 106A to 109A, 106B to 109B, 106C to 109C and 106D to 109D arein an orthogonal relationship. The number of unit preambles LP 106-109for each transmit antenna is equal to the number of transmit antennas inMIMO mode. In order to distinguish between two kinds of long-preamblesequences, first long-preamble sequence 102 conforming to IEEE 802.11amay be called “legacy long training field”. Since the second longpreambles sequences 106-109 are provided for new high throughputwireless LAN standard, it may be called “high throughput long trainingfield”.

After each of the second long-preamble sequences 106A to 109A, 106B to109B, 106C to 109C and 106D to 109D, a field for transmission data(DATA) 110A to 110C transmitted from the antennas Tx1 to Tx4,respectively, is positioned. The second long-preamble sequences 106A to109A, 106B to 109B, 106C to 109C and 106D to 109D are transmittedsimultaneously from a plurality of antennas Tx1 to Tx4 respectively.

Referring now to FIG. 2, the wireless transmitting device according tothe embodiment will be described. Firstly, digital modulator 203 forms asignal for wireless packet by combining transmission data 201 and theabove-described preamble outputted from a memory 202. The thus-obtainedsignal for wireless packet is sent to transmitting units 204A to 204D,where they are subjected to processing needed for transmission, forexample, digital-to-analog (D/A) conversion, frequency conversion into aradio frequency (RF) band (up-conversion) and power amplification.Thereafter, the resultant signal is sent to a plurality of antennas 205Ato 205D corresponding to the antennas Tx1 to Tx4 described withreference to FIG. 1, where an RF signal is sent from each transmitantenna 205A to 205D to the wireless receiving device shown in FIG. 3.In the description below, the antennas Tx1 to Tx4 shown in FIG. 1 arereferred to as the antennas 205A to 205D, respectively.

In the embodiment, the PLCP signal shown in FIG. 1, which includes theshort-preamble sequence 101, first long-preamble sequence 102, firstsignal field 103 and second signal field 104, is transmitted from thetransmit antenna 205A of the transmission unit 204A shown in FIG. 2. TheAGC preambles 105A to 105D, second long-preamble sequences 106A to 109A,106B to 109B, 106C to 109C and 106D to 109D, which are positioned afterthe PLCP signal as shown in FIG. 1, and the data 110A to 110D aretransmitted across all the transmit antennas 205A to 205D.

In the wireless receiving device shown in FIG. 3, a plurality ofreceiving antennas 301A to 301D receive RF signals transmitted from thewireless transmitting device shown in FIG. 2. The wireless receivingdevice may have one receiving antenna or multiple receiving antennas.The RF signals received by the receiving antennas 301A to 301D are sentto receiving units 302A to 302D, respectively. The receiving units 302Ato 302D each perform various types of receiving processing, such asfrequency conversion (down-conversion) from the RF band to BB(baseband), automatic gain control (AGC), analog-to-digital conversion,etc., thereby generating a baseband signal.

The baseband signals from the receiving units 302A to 302D are sent tochannel impulse response estimation units 303A to 303D and digitaldemodulator 304. These units 303A to 303D estimate the impulse responsesof the respective propagation paths between the wireless transmittingdevice of FIG. 2 and the wireless receiving device of FIG. 3. Thechannel impulse response estimation units 303A to 303D will be describedlater in detail. The digital demodulator 304 demodulates the basebandsignals based on the estimated channel impulse response provided byunits 303A to 303D, thereby generating received data 305 correspondingto the transmission data 201 shown in FIG. 2.

More specifically, the digital demodulator 304 has an equalizer of thechannel impulse response at its input section. The equalizer performsequalization for correcting the received signal distorted in thepropagation path, based on the estimated channel impulse response. Thedigital demodulator 304 also demodulates the equalized signal atappropriate timing determined by the time synchronization, therebyreproducing data.

The receiving units 302A to 302D shown in FIG. 3 will now be described.FIG. 4 shows the configuration of the receiving unit 302A in detail.Since the other receiving units 302B to 302D have the same configurationas the unit 302A, only the receiving unit 302A will be described. The RFreceived signal received by the receiving antenna 301A is down-convertedby a down-converter 401 into a baseband signal. At this time, The RFsignal may be directly converted into a baseband signal, or may befirstly converted into an intermediate frequency (IF) signal and theninto a baseband signal.

The baseband signal generated by the down-converter 401 is sent to avariable gain amplifier 402, where it is subjected to perform AGC, i.e.,signal level adjustment. The signal output from the variable gainamplifier 402 is sampled and quantized by an A/D converter 403. Thedigital signal output from the A/D converter 403 is sent to the outsideof the receiving unit 302 and to a gain controller 404. The gaincontroller 404 performs gain calculation based on the digital signaloutput from the A/D converter 403, and controls the gain of the variablegain amplifier 402. The specific procedure for the gain control will bedescribed later.

The operation of the wireless receiving device shown in FIGS. 3 and 4executed for receiving the wireless packet including the preamble whoseformat is shown in FIG. 1 is as follows. Firstly, the wireless receivingdevice receives a short-preamble sequence 101 transmitted from thetransmit antenna 205A of FIG. 2, and then performs packet edgedetection, time synchronization, auto frequency control (AFC) and AGC,using a baseband signal corresponding to the short-preamble sequence101. AFC is also called frequency synchronization. Packet edgedetection, time synchronization and AFC can be performed using knowntechniques, therefore no description will be given thereof. Only AGCwill be explained below.

The baseband signal corresponding to the short-preamble sequence 101 isamplified by the variable gain amplifier 402 in accordance with apredetermined initial gain value. The signal output from the variablegain amplifier 402 is input to the gain controller 404 via the A/Dconverter 403. The gain controller 404 calculates a gain from the levelof the received signal corresponding to the short-preamble sequence 101,which is acquired after A/D conversion, and controls the gain of thevariable gain amplifier 402 in accordance with the calculated gain.

Assume here that the level of the baseband signal corresponding to theshort-preamble sequence 101, which is acquired before A/D conversion, isX. If level X is high, the baseband signal input to the A/D converter403 exceeds the upper limit of the input dynamic range of the A/Dconverter 403. As a result, the signal (digital signal) output from theA/D converter 403 is saturated and degraded the quality of signalreception. On the other hand, if level X is extremely low, the signaloutput from the A/D converter 402 (i.e., the digital signal acquired byA/D conversion) suffers a severe quantization error. Thus, when level XL is very high or low, the A/D converter 403 cannot perform appropriateconversion, thereby significantly degrading the quality of signalreception.

To overcome this problem, the gain controller 404 controls the gain ofthe variable gain amplifier 402 so that the level X of the basebandsignal corresponding to the short-preamble sequence 101, is adjusted toa target value Z. If the input baseband signal has such a very highlevel as makes the output of the A/D converter 403 limited to its upperlimit level, or if it has a very low level, the gain of the variablegain amplifier 402 may not appropriately be controlled by one controlprocess. In this case, gain control is performed repeatedly. As aresult, the level of the baseband signal input to the A/D converter 403can be adjusted to a value that falls within the input dynamic range ofthe A/D converter 403. Thus, the gain of the variable gain amplifier 402is appropriately controlled using the baseband signal corresponding tothe short-preamble sequence 101, thereby performing appropriate A/Dconversion to avoid a reduction in the quality of signal reception.

In the above-described embodiment, the reception level needed forcalculating the gain of the variable gain amplifier 402 is measuredusing a digital signal output from the A/D converter 403. However, suchlevel measurement can be executed using an analog signal acquired beforeA/D conversion. Furthermore, the reception level may be measured in theIF band or RF band, instead of BB.

The wireless receiving device receives a first long-preamble sequence102 transmitted from the transmit antenna 205A, and performs theestimation of channel impulse response, i.e., estimates the response(frequency transfer function) of the propagation path between thewireless transmitting device to the wireless receiving device, using abaseband signal corresponding to the long-preamble sequence 102. Sincethe signal transmitted from the transmit antenna 205A has already beensubjected to AGC as described above, the level of an input to the A/Dconverter 403 is appropriately adjusted when the estimation of channelimpulse response is performed. Accordingly, concerning the signaltransmitted from the transmit antenna 205A, a highly accurate digitalsignal is acquired from the A/D converter 403. The estimation of channelimpulse can be performed accurately with the acquired digital signal.

The wireless receiving device receives a first signal field 103transmitted from the transmit antenna 205A, and demodulates a basebandsignal corresponding to the first signal field 103, using the digitaldemodulator 304 and the above-mentioned propagation path estimationresult. The first signal field 103 contains information indicating themodulation scheme and wireless packet length of data to be sent afterthe preamble. The first signal field 103 is a field that conveys a kindof attribute information regarding the wireless packet. The wirelessreceiving device continues demodulation using the digital demodulator304 during the duration of a wireless packet recognized from thewireless packet length information contained in the first signal field103.

Since the packet format from the short-preamble sequence 101 to thefirst signal field 103 provides interoperability with IEEE802.11astations, IEEE 802.11a station is able to perform normal receivingoperation without destroying the wireless packet. In other words,another IEEE 802.11a wireless transmitting and receiving deviceconforming to the IEEE 802.11a standard (a legacy station), uponreceiving the first signal field 103, is prohibited to transmit a signaluntil the wireless packet ends so as not to destroy the wireless packet.

Subsequently, the wireless receiving device receives a second signalfield 104 transmitted from the transmit antenna 205A. The second signalfield 104 contains identification information indicating a wirelesspacket that corresponds to a standard other than IEEE 802.11a, e.g.,IEEE 802.11n. In other words, the second signal field 104 indicates thatsubsequent AGC preambles 105A to 105D, second long-preamble sequences106A to 109A, 106B to 109B, 106C to 109C and 106D to 109D are signalscorresponding to, for example, IEEE 802.11n.

The wireless receiving device receives AGC preambles 105A to 105Dtransmitted from the transmit antennas 205A to 205D in parallel. The AGCpreambles 105A to 105D are transmitted from the transmit antenna 205Athat has transmitted the short-preamble sequence 101, firstlong-preamble sequence 102, first signal field 103 and second signalfield 104, and from the transmit antennas 205B to 205D that havetransmitted no signal so far. Accordingly, while the signals transmittedfrom the transmit antenna 205A (i.e., the short-preamble sequence 101,first long-preamble sequence 102, first signal field 103 and secondsignal field 104) are received with a certain receiving level, the AGCpreambles 105A to 105D are received with different receiving levels fromthe level of the reception signal coming from the transmit antenna 205A.In other words, the reception level is changed after the MIMOtransmission using the multiple transmit antenna.

As described above, the wireless receiving device receives the secondsignal field 104 and demodulates it using the digital demodulator 304,thereby recognizing that the present wireless packet corresponds to IEEE802.11n. After that, the digital demodulator 304 issues an instructionto restart AGC for fine tune to the receiving units 302A to 302D,thereby re-executing AGC on the AGC preambles 105A to 105D. As a result,the signals transmitted from the transmit antennas 205A to 205D via theMIMO channel and received at the receiving units 302A to 302D, are inputto the A/D converter 403 with an appropriately adjusted receiving level.

That is, using the level of baseband signals corresponding to the AGCpreambles 105A to 105D, which is acquired after A/D conversion as shownin FIG. 4, gain control is performed on the variable gain amplifier 402.The time at which the digital demodulator 304 issues the instruction tostart AGC using the AGC preambles 105A to 105D is not limited to thetime at which the decoding result of the second signal field 104 isacquired. For instance, the digital demodulator 304 may confirm, using,for example, a matched filter, the reception of the AGC preambles 105Ato 105D, and then supply the receiving units 302A to 302D with aninstruction to start AGC.

In the preamble proposed by Jan Boer, which is described before, AGC isperformed only using a short-preamble sequence (legacy short preamble),transmitted from a single transmit antenna. AGC is performed using areception level with which the signal transmitted from the antenna wherethe short-preamble sequence transmits. When a wireless receiving devicereceives signals transmitted from other three antennas, the deviceexecutes gain control by using the acquired gain.

FIG. 5 is a graph illustrating the distribution of the receiving powerof a short preamble and data, acquired when Jan Boer's proposed preambleis utilized. The channel is in a multipath environment with a delayspread of 50 nsec (the duration for one data symbol is 4 μsec). As isevident from this figure, the ratio of the receiving level of shortpreamble (legacy short preamble) to the receiving level of the datavaries significantly.

In, for example, region A in FIG. 5, the short preamble is received witha high receiving level, although the receiving level of data is low.Accordingly, if AGC is adjusted in accordance with the receiving powerof the short preamble, the receiving power of the data is lower than thereceiving power of the short preamble, resulting in a quantization errorin the A/D converter 403. In region B in FIG. 5, the short preamble isreceived with a low receiving level, although the receiving level ofdata is high. Accordingly, if AGC is adjusted in accordance with thereceiving power of the short preamble, the output of the A/D converterwhen data is input is saturated. Thus, it is understood that since, inthe conventional scheme, the receiving power ratio of data to theshort-preamble is not constant; the receiving characteristic is degradedbecause of a quantization error or saturation in the output of the A/Dconverter.

On the other hand, in the embodiment, all antennas 205A to 205D thattransmit data signals transmit AGC preambles 105A to 105D, respectively.FIG. 6 shows the distribution of the receiving power of theshort-preambles and data, according to the embodiment. The channelenvironment is the same as in the case of FIG. 5.

As shown in FIG. 6, the receiving power of the AGC preambles issubstantially proportional to that of the data 110A to 110D. Thisindicates that the input level of the A/D converter is adjusted soappropriate that the receiving accuracy is remarkably enhanced ascompared to the FIG. 5.

FIG. 7 shows a modification of the receiving unit 302A. In general, todetect an unknown signal, the variable gain amplifier 402 uses arelatively large gain as the initial value. Accordingly, if the gain ofthe variable amplifier 402 is initialized when the AGC preambles 105A to105D are received, it is necessary to repeat gain control until the gainis stabilized. The modification shown in FIG. 7 provides a memory 405.This memory 405 stores the gain value acquired after the AGC wasexecuted with the short-preamble sequence 101. When receiving the AGCpreambles 105A to 105D, if the gain of the amplifier 402 is not returnedto the initial value set in the standby state, but the gain read fromthe memory 405 is used as its initial value, AGC can be performed notonly accurately but also finished in a short time compare to the casewithout using such stored value.

Referring then to the flowchart of FIG. 8A, the operation of the gaincontroller 404 will be described in detail.

Upon receiving the head of the short-preamble sequence 101, thereceiving device starts AGC (step S1).

Subsequently, zero is set as a counter value (i) (step S2).

Subsequently, referring to the counter value, it is determined whetherAGC is in the initial stage or middle stage (step S3). At this time,since the counter value is zero, the answer to the question at step S3is YES, thereby proceeding to step S4.

After that, it is determined whether the preamble 105 is now beingreceived (step S4). In this case, since the short-preamble sequence 101as the head of a wireless packet is being received, the answer to thequestion at step S4 is NO, thereby proceeding to step S5. At step S5, apredetermined initial value is set.

At the next step S6, the amplification factor of the variable gainamplifier is changed in accordance with the set initial value. At thenext step S7, the receiving level of the present short-preamble sequenceis measured. It is determined at step S8 whether the measured level isan appropriate level (target level) for the A/D converter. If the answerto the question at step S8 is NO, the program proceeds to step S9.

At step S9, the counter value is implemented, and then the programreturns to step S3. At step S3, it is determined that i is not zero, theprogram proceeds to step S10. At step S10, gain calculation is performedusing the level measured at step S7.

Thus, the loop of S10→S6→S7→S8→S9 is repeated until the receiving levelreaches the target level. When the receiving level has reached thetarget level, the set gain is written to the memory 405 at step S11,thereby finishing AGC performed on the signal transmitted from theantenna Tx1. This AGC operation (first AGC) plays a role as “a coarseAGC” at the receiving device by contrast with the next fine AGCoperation (second AGC) for MIMO reception using the AGC preambles 105which will be described later.

The receiving unit 302A then receives the long-preamble sequence 102,first signal field 103 and second signal field 104. The receiving unit302A starts AGC for MIMO reception with the AGC preambles 105. AGCstarts from step S1, and shifts to S2, S3 and S4. At step S4, since thereceiving unit 302A is receiving the AGC preambles 105, the programproceeds to step S12, thereby reading the gain value previously writtento the memory 405 and followed by step S6. After step S6, the sameprocess as the above is performed.

The flow discussed above is summarized as follows. The summarized flowchart is shown in FIG. 8B. First, receive the short-preamble sequence101 at wireless receiving device (step S21). Then, start the first AGCoperation (step S22) and set a gain for variable gain amplifiers 402A to402D (step S23). Then, write the set gain to the memory 405 (step S24).After the first AGC operation, then start the second AGC operation withthe result of the reception of the AGC preambles 105A to 105Dtransmitted from multiple transmit antennas by using MIMO technique(step S25). Then, refer to the gain written in the memory 405 (step S26)and set new gain for each of variable gain amplifiers 402A to 402D (step27).

Thus, when receiving the AGC preambles 105A to 105D, the gain is notreturned to the initial value set in the standby state, but the gain,which is acquired by the first AGC, stored in the memory 405 is used asthe initial value. Because of this operation, the AGC preambles 105A to105D enables the wireless receiving device to perform fine AGC in MIMOreception with a short time period. This fine AGC provides sufficientaccuracy for the MIMO reception.

FIG. 9 is a view illustrating a modification of the wireless receivingdevice of FIG. 3, in which AGC is commonly performed. FIG. 9 differsfrom FIG. 3 in which in the former, a common receiving unit 302 isprovided for the antennas 301A to 301D.

FIG. 10 shows the receiving unit 302 of FIG. 9 in detail. Theconfiguration of FIG. 10 differs from that of FIG. 7 in that in theformer, a single gain controller 404 and a memory 405 for storing a gainvalue acquired using the short-preamble sequence 101 are commonlyprovided for the antennas 301A to 301D.

Specifically, the output signals of the antennas 301A to 301D are inputto A/D converters 403A to 403D via down-converters 401A to 401D andvariable gain amplifiers 402A to 402D, respectively. The output signalsof the A/D converters 403A to 403D are input to the common gaincontroller 404. The gain determined by the gain controller 404 iscommonly input to the variable gain amplifiers 402A to 402D. Forexample, the gain, which enables the highest one of the levels acquiredafter A/D conversion by the A/D converters 403A to 403D to be set as atarget Z, may be commonly input to the variable gain amplifiers 402A to402D.

Also in the receiving device shown in FIGS. 9 and 10, the digitaldemodulator 304 confirms the reception of the short-preamble sequence101 and supplies the receiving unit 302 with an instruction to start thefirst AGC. After that, the digital demodulator 304 confirms thereception of the second signal field 104 or AGC preambles 105, andsupplies the receiving unit 302 with an instruction to start the secondAGC for MIMO reception mode.

Thereafter, the wireless receiving device receives the secondlong-preamble sequences 106A to 109A, 106B to 109B, 106C to 109C and106D to 109D, which are transmitted after the AGC preambles 105A to 105Dfrom the transmission antennas 205A to 205D. The unit preambles LP thatform the second long-preamble sequences 106A to 109A, 106B to 109B, 106Cto 109C and 106D to 109D are basically the same signals as those formingthe first long-preamble sequence 102.

Further, the second long-preamble sequences 106A to 109A, 106B to 109B,106C to 109C and 106D to 109D are signals subjected to orthogonalizationusing Walsh sequences. In other words, in FIG. 1, each unit preamblewith symbol “−LP” has a polarity reverse to that of each unit preamblewith symbol “LP”. The wireless receiving device receives the secondlong-preamble sequences 106A to 109A, 106B to 109B, 106C to 109C and106D to 109D, which are synthesized with each other. As will bedescribed later, the signals transmitted from the transmit antennas 205Ato 205D are reproduced by multiplying the second long-preamble sequencesby Walsh sequences.

A detailed description will be given of the channel impulse responseestimation units 303A to 303D. FIG. 11 illustrates the channel impulseresponse estimation unit 303A in detail. Since the other estimationunits are similar to the estimation unit 303A, only the estimation unit303A will be described. The channel impulse response estimation unit303A comprises estimation units 501A to 501D for estimating theresponses of the propagation paths between the receiving antenna 301Aand the antennas Tx1 to Tx4 (corresponding the transmit antennas 205A to205D) of a wireless transmitting device, respectively.

The estimation unit 501A includes data memories 502A to 502D for storingthe respective symbol of the received second long-preamble sequence,coefficient memories 503A to 503D for storing respective coefficients bywhich the respective symbol of the received second long-preamblesequence is be multiplied, multipliers 504A to 504D and an adder 505.The other estimation units 501B to 501D have the same structure as theestimation unit 501A, except for the value of the coefficients by whichthe respective symbols of the received second long-preamble sequences isbe multiplied. The data memories 502A to 502D are connected in series,thereby forming a shift register.

In the estimation unit 501A, the received second long-preamble sequences106A to 109A, 106B to 109B, 106C to 109C and 106D to 109D are stored inthe data memories 502A to 502D. Specifically, the memory 502A stores thevalue of the signal acquired by combining the long-preamble sequence106A to 106D included in the second long-preamble sequences. Similarly,the memory 502B stores the value of the signal acquired by combining thelong-preamble sequence 107A to 107D, the memory 502C stores the value ofthe signal acquired by combining the long-preamble sequence 108A to108D, and the memory 502D stores the value of the signal acquired bycombining the long-preamble sequence 109A to 109D.

Assuming that the responses of the propagation paths between thetransmit antennas 205A to 205D and the receiving antenna 301A are h1,h2, h3 and h4, signal values S_(502A), S_(502B), S_(502C) and S_(502D)stored in the data memories 502A, 502B, 502C and 502D, respectively, aregiven byS _(502A)=LP*h ₁+LP*h ₂+LP*h ₃+LP*h ₄  (1)S _(502B)=LP*h ₁+LP*h ₂−LP*h ₃−LP*h ₄  (2)S _(502C)=LP*h ₁−LP*h ₂−LP*h ₃+LP*h ₄  (3)S _(502D)=LP*h ₁−LP*h ₂+LP*h ₃−LP*h ₄  (4)

The multipliers 504A, 504B, 504C and 504D multiply the signal values,stored in the data memories 502A, 502B, 502C and 502D, by thecoefficients stored in the coefficient memories 503A, 503B, 503C and503D, respectively. In the estimation unit 501A, a coefficient of 1 isstored in all coefficient memories 503A, 503B, 503C and 503D for theestimation of channel impulse response between the transmit antenna 205Aand the receiving antenna 301A. That is, the coefficients stored in thecoefficient memories 503A, 503B, 503C and 503D are expressed by asequence of (1, 1, 1, 1).

Thereafter, the adder 505 adds the multiplication results of themultipliers 504A to 504D. In this case, the signal values S_(502A),S_(502B), S_(502C) and S_(502D) given by the equations (1) to (4) areadded. As is evident from the equations (1) to (4), only the longpreamble PL and the value h1 that indicates the channel impulse responsebetween the antenna Tx1 (transmit antenna 205A) and the receivingantenna remain as the addition result. If unit preambles PL that form along-preamble sequence are each provided as a predetermined bit patternfor the wireless transmitting device and wireless receiving device, thechannel impulse response between the transmit antenna 205A and thereceiving antenna 301A can be estimated from the received signalacquired by combining the signals transmitted from all transmit antennas205A to 205D.

On the other hand, in the estimation units 501B, 501C and 501D, thecoefficient memories 503B, 503C and 503D store Walsh sequences of (1, 1,−1, −1), (1, −1, −1, 1) and (1, −1, 1, −1), respectively. As a result,the estimation units 501B, 501C and 501D can estimate the channelimpulse response between the antennas Tx2, Tx3 and Tx4 (transmitantennas 205B, 205C and 205D) and the receiving antenna 301A,respectively.

As described above, the channel impulse response estimation unit 303Aestimates the response of the propagation path between each of thetransmit antennas 205A to 205D and the receiving antenna 301A.Similarly, the channel impulse response estimation units 303B to 303Cestimate the channel impulse response between the transmit antennas 205Ato 205D and the receiving antennas 301B to 301C.

In AGC using the AGC preambles 105A to 105D, gain control is performedusing, as an initial value, the value of the gain of the variable gainamplifier 402 adjusted using a signal transmitted from a singletransmitting antenna 205A, with the result that fine and fast gaincontrol can be achieved. Examples of the AGC preambles 105A to 105D willnow be described. The AGC preambles 105A to 105D shown in FIGS. 12( a),(b), (c) and (d) are each formed of a signal sequence including aplurality of time-domain samples (ten samples in the case of FIG. 12).The AGC preamble 105A transmitted from the antenna Tx1, for example,comprises a sequence of (a0, a1, a2, . . . , a8, a9).

Further, the AGC preambles 105A to 105D shown in FIGS. 12( a), (b), (c)and (d) are formed by cyclic shifting the samples in time domain of asingle signal sequence. Specifically, a signal sequence acquired bycyclic shifting of the samples in time domain of an AGC preamblesequence transmitted from a certain reference antenna is an AGC preamblesequence transmitted from another antenna. For example, the AGC preamblesequence 105B transmitted from the antenna Tx2 is (a1, a2, . . . , a9,a0), which is acquired by cyclic shifting, by one sample, the temporalpositions of the samples of the AGC preamble 105A transmitted from thereference antenna Tx1.

Similarly, the AGC preamble 105C transmitted from the antenna Tx3 isacquired by cyclic shifting, by two samples, the temporal positions ofthe samples of the AGC preamble 105A transmitted from the referenceantenna Tx1. The AGC preamble 105D transmitted from the antenna Tx4 isacquired by cyclic shifting, by three samples, the temporal positions ofthe samples of the AGC preamble 105A transmitted from the antenna Tx1 asreference.

If the AGC preambles 105A to 105D are formed of signal sequencesidentical to each other, they may well interfere with each other duringtransmission. Such interference may cause an electric field similar tothat occurring when directional antenna transmission is performed,depending upon a multipath state or receiving point. As a result, a nullpoint may occur. In other words, there may occur a receiving point atwhich none of the AGC preambles can be received and the receiving levelmay not accurately be measured.

In the embodiment, a multipath formed of signal sequences (i.e., the AGCpreambles 105A to 105D) that are acquired by cyclic shifting thetemporal positions of their samples is intentionally created. In thiscase, even if the receiving level of a certain sample in the signalsequences is reduced because of signal interference, the probability ofoccurrence of a reduction in the receiving level of another sample islow. Therefore, accurate receiving level measurement is realized, whichenhances the receiving performance of the wireless receiving device. Forinstance, a communication system can be realized which is not against aprotocol, CSMA/CA (Carrier Sense Multiple Access with CollisionAvoidance), stipulated in IEEE, 802.11.

FIG. 13( a) to (d) show other examples of the AGC preambles 105A to105D. The AGC preambles 105A to 105D shown in FIG. 12( a) to (d) aretime-domain signal sequences acquired by cyclic shifting the temporalpositions of their samples to each other. On the other hand, the AGCpreambles 105A to 105D shown in FIG. 13( a) to (d) are frequency-domainsignal sequences, and have different frequency components. In FIG. 13,f0 to f15 indicate subcarrier frequencies, and the hatched subcarrierscarry signals, while non-hatched subcarriers do not carry signals.

For example, the AGC preamble 105A transmitted from the antenna Tx1 isformed of subcarriers f0, f4, f8 and f12. Similarly, the AGC preamble105B transmitted from the antenna Tx2 is formed of subcarriers f1, f5,f9 and f13. The AGC preamble 105C transmitted from the antenna Tx3 isformed of subcarriers f2, f6, f10 and f14. Further, the AGC preamble105D transmitted from the antenna Tx4 is formed of subcarriers f3, f7,f11 and f15. The subcarriers transmitted from the antenna Tx1 are notsent by any other antenna. Similarly, the subcarriers transmitted fromthe antenna Tx2 are not sent by any other antenna.

Actually, the AGC preambles 105A to 105D are transmitted after they aretransformed into time-domain signal sequences by inverse fast Fouriertransform (IFFT) or discrete Fourier transform (DFT). In the wirelesstransmitting device, as shown in FIG. 14, a memory 202 stores, as AGCpreambles, data concerning the frequency-domain signal sequences asshown in FIG. 13( a) to (d). The frequency-domain signal sequence dataread from the memory 202 is transformed into time-domain signalsequences by an IFFT circuit 206, and input to a digital modulator 203.The digital modulator 203 may incorporate the function of the IFFTcircuit 206. Furthermore, the memory 202 may pre-store time-domainsignal sequence data into which the frequency-domain signal sequencedata shown in FIG. 13( a) to (d) is transformed. In this case, the IFFTcircuit 206 is not needed.

As shown in FIG. 13( a) to (d), since the AGC preambles 105A to 105D arefrequency interleaved across four antennas, the signals from theantennas Tx1 to Tx4 do not contain the same frequency component,therefore can reach the wireless receiving device without interferingwith each other. As a result, the wireless receiving device can performaccurate receiving level measurement and hence exhibit high receivingperformance.

The present invention is not limited to the above-described embodiments,but may be modified in various ways without departing from the scope.For instance, in the embodiments shown in FIG. 2, digital-to-analog(D/A) conversion is performed in transmission units 204A to 204Drespectively. But, it can be modified that digital modulator 203performs such D/A conversion instead of the transmission units 204A to204D. Similarly, the embodiments shown in FIG. 3, analog-to-digital(A/D) conversion is performed in receiving units 302A to 302Drespectively. But, it can be modified that such A/D conversion isperformed by digital demodulator 304 instead of the units 302A to 302D.

With regard to the packet format, the short-preamble sequence 101, firstlong-preamble sequence 102, first signal field (SIGNAL) 103 and secondsignal field (SIGNAL 2) 104 are transmitted from antenna Tx1 as shown inFIG. 1. But, it can be possible that such preamble signal is transmittedfrom at least one transmitted antenna. It is possible that each of thesecond long-preamble sequences may have different frequency componentslike the AGC preambles 105A to 105D shown in FIG. 13( a) to (d).

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A wireless transmitting device for communicating with a wirelessreceiving device by using a wireless packet, the wireless transmittingdevice comprising: a signal generator configured to generate a signalfor transmitting the wireless packet, the wireless packet including: ashort-preamble used for a first automatic gain control (1st AGC) at thewireless receiving device, a first long-preamble used for an estimationof a first channel impulse response between the wireless transmittingdevice and the wireless receiving device, a signal field following theshort-preamble and the first long-preamble, and used for conveyinginformation regarding the wireless packet, an AGC preamble following thesignal field, and used for a second automatic gain control (2nd AGC)performed after the first automatic gain control at the wirelessreceiving device, a second long-preamble used for an estimation of asecond channel impulse response between the wireless transmitting deviceand the wireless receiving device, and a data field conveying data;wherein the AGC preamble and the data field are transmitted in parallelvia a plurality of antennas.
 2. The wireless transmitting deviceaccording to claim 1, wherein the data field is transmitted via theantennas.
 3. The wireless transmitting device according to claim 1,wherein the short-preamble, the first long-preamble, and the signalfield are transmitted via at least one of the antennas.
 4. The wirelesstransmitting device according to claim 1, wherein the short-preamble,the first long-preamble, and the signal field are transmitted via atleast one of the antennas, the short-preamble, the first long-preamble,and the signal field are recognizable by both a first communicationsystem and a second communication system; and the AGC preamble and thedata field are recognizable by the second communication system.
 5. Thewireless transmitting device according to claim 4, wherein the secondcommunication system has backwards compatibility with the firstcommunication system.
 6. The wireless transmitting device according toclaim 4, wherein the second communication system has interoperabilitywith the first communication system.
 7. The wireless transmitting deviceaccording to claim 4, wherein the first communication system conforms toIEEE802.11a, and the second communication system conforms toIEEE802.11n.
 8. The wireless transmitting device according to claim 1,wherein cyclic shifting of temporal positions is applied to the AGCpreamble.
 9. The wireless transmitting device according to claim 1,wherein the signal generator is configured to generate the signal fortransmitting the wireless packet that includes the AGC preamble that istransmitted via the antennas, which use frequency components.
 10. Thewireless transmitting device according to claim 1, wherein the signalgenerator is configured to generate the signal for transmitting thewireless packet that includes the signal field, which comprises: a firstsignal field conforming to IEEE 802.11a; and a second signal fieldindicating that the AGC preamble and the second long-preamble conform toa standard other than IEEE 802.11a.
 11. The wireless transmitting deviceaccording to claim 1, wherein the signal generator is configured togenerate the signal for transmitting the wireless packet that includesthe second long-preamble, which is transmitted by the antennas and isorthogonalized with respect to another second long-preamble by usingWalsh sequences.
 12. The wireless transmitting device according to claim1, wherein the information is regarded as a length of the data field.13. A wireless transmitting method for using a wireless transmittingdevice for communicating with a wireless receiving device by using awireless packet, the method comprising: generating a signal fortransmitting the wireless packet, the wireless packet including: ashort-preamble used for a first automatic gain control (1st AGC) at thewireless receiving device, a first long-preamble used for an estimationof a first channel impulse response between the wireless transmittingdevice and the wireless receiving device, a signal field following theshort-preamble and the first long-preamble, and used for conveyinginformation regarding the wireless packet, an AGC preamble following thesignal field, and used for a second automatic gain control (2nd AGC)performed after the first automatic gain control at the wirelessreceiving device, a second long-preamble used for an estimation of asecond channel impulse response between the wireless transmitting deviceand the wireless receiving device, and a data field conveying data; andtransmitting the AGC preamble and the data field in parallel via aplurality of antennas.
 14. The wireless transmitting method according toclaim 13, further comprising: transmitting the short-preamble, the firstlong-preamble, and the signal field via at least one of the antennas.15. The wireless transmitting method according to claim 13, furthercomprising: transmitting the short-preamble, the first long-preamble,and the signal field via at least one of the antennas, wherein theshort-preamble, the first long-preamble, and the signal field arerecognizable by both a first communication system and a secondcommunication system; and the AGC preamble and the data field arerecognizable by the second communication system.
 16. The wirelesstransmitting method according to claim 13, wherein the information isregarded as a length of the data field.
 17. A wireless transmittingdevice for communicating with a wireless receiving device by using awireless packet, the wireless transmitting device comprising: aplurality of antennas; and a signal generator configured to generate asignal for transmitting the wireless packet, the wireless packetincluding: a short-preamble used for a first automatic gain control (1stAGC) at the wireless receiving device, a first long-preamble used for anestimation of a first channel impulse response between the wirelesstransmitting device and the wireless receiving device, a signal fieldfollowing the short-preamble and the first long-preamble, and used forconveying information regarding the wireless packet, an AGC preamblefollowing the signal field, and used for a second automatic gain control(2nd AGC) performed after the first automatic gain control at thewireless receiving device, a second long-preamble used for an estimationof a second channel impulse response between the wireless transmittingdevice and the wireless receiving device, and a data field conveyingdata; wherein the AGC preamble and the data field are transmitted inparallel via the antennas.
 18. The wireless transmitting deviceaccording to claim 17, wherein the short-preamble, the firstlong-preamble, and the signal field are transmitted via at least one ofthe antennas.
 19. The wireless transmitting device according to claim17, wherein the short-preamble, the first long-preamble, and the signalfield are transmitted via at least one of the antennas, theshort-preamble, the first long-preamble, and the signal field arerecognizable by both a first communication system and a secondcommunication system; and the AGC preamble and the data field arerecognizable by the second communication system.
 20. The wirelesstransmitting device according to claim 19, wherein the secondcommunication system has backwards compatibility with the firstcommunication system.
 21. The wireless transmitting device according toclaim 19, wherein the second communication system has interoperabilitywith the first communication system.
 22. The wireless transmittingdevice according to claim 19, wherein the first communication systemconforms to IEEE802.11a, and the second communication system conforms toIEEE802.11n.
 23. The wireless transmitting device according to claim 17,wherein the signal generator is configured to generate the signal fortransmitting the wireless packet that includes the AGC preamble, whichis formed by cyclic shifting of temporal positions, and an amount ofcyclic shift is different for different ones of the antennas.
 24. Thewireless transmitting device according to claim 17, wherein the signalgenerator is configured to generate the signal for transmitting thewireless packet that includes the AGC preamble that is transmitted viathe antennas, which use frequency components.
 25. The wirelesstransmitting device according to claim 17, wherein the signal generatoris configured to generate the signal for transmitting the wirelesspacket that includes the signal field, which comprises: a first signalfield conforming to IEEE 802.11a; and a second signal field indicatingthat the AGC preamble and the second long-preamble conform to a standardother than IEEE 802.11a.
 26. The wireless transmitting device accordingto claim 17, wherein the signal generator is configured to generate thesignal for transmitting the wireless packet that includes the secondlong-preamble, which is transmitted by the antennas and isorthogonalized with respect to another second long-preamble by usingWalsh sequences.
 27. The wireless transmitting device according to claim17, wherein the information is regarded as a length of the data field.